“A lot of times you’re buying a neighborhood,” says John Bredemeyer, a spokesman for the Chicago-based Appraisal Institute, the largest professional organization of real estate appraisers in the U.S. “When a lot of people want in, the price per square foot a seller can get goes up.”

All About Amenities

If you understand nothing else in real estate, understand that an apartment building’s location, amenities and access to grocery stores, transportation and schools have a direct impact on values.

Within the same neighborhood, differentiating factors like outdoor space, proximity to health clubs and whether or not the building has doormen factor into a unit’s cost per square foot.

What’s more, a few lawns amidst an urban concrete jungle can be invaluable to buyers. Many of the homes on our list are penthouses–properties that are both rare and possess a great deal of exterior space. In New York, in particular, lack of outdoor space, coupled with calculation definitions for square footage, make a terrace one of the most desired of apartment amenities.

“What you’re really paying for with a penthouse is the outdoor space,” says Joel Silver, a designer of the Tribeca Summit’s $32 million penthouse in Manhattan. “[Outdoor space] is harder to find, is more prized and isn’t included in square footage.”

Brand-Name Buildings

Then there are the starchitects.

Recently, developers in geographically bounded metropolitan areas like New York, Seattle or San Francisco have used name-brand architects as a vehicle to increase an apartment’s value per square foot. So-called starchitects like Richard Meier, Norman Foster and Frank Gehry greatly bolster asking prices, experts say.

“It varies case by case, but a starchitect-designed building can get double what a regular building next door would receive,” says Jonathan Miller, president of Miller Samuel, a New York-based real estate appraisal and consultancy firm. “These types of projects usually break neighborhood records and [particular starchitects] have developed track records which justify their fees.”

Most starchitect efforts are new constructions, where building costs–dependent on labor, materials, fees and transportation costs–are passed along to the consumer. The more expensive each square foot costs to build, the more expensive the sales price.

In a market like Boston, the costs associated with building are 26% higher than the national average of building new constructions, according to Marshall & Swift, a building-costs research firm in Los Angeles. Manhattan, as a market, costs even more. Building there costs 46% more than the national average. Naturally, costs per square foot will follow.

Foreign Influx

Whether a Gehry-designed building or a penthouse with ocean views, many luxury apartments are sold to out-of-town buyers–especially the case in financial centers like New York, commerce centers like Houston or entertainment capitals like Los Angeles. This has a multiplying effect on neighborhood values, as an out-of-town or foreign buyer does not sell a local property in order to buy a new one. Demand goes up but, with no inventory freed up, prices accelerate.

Such a phenomenon is best observed in London. The steady flow of Arab princes and Russian oligarchs into neighborhoods like Kensington and Knightsbridge has resulted in some of the world’s most valuable apartments per square foot, including the sale of One Hyde Park, the world’s most expensive apartment.

“The U.S. dollar’s exchange rate is down,” says Peter Kozel, executive managing director of Newmark Knight Frank, the North American arm of worldwide appraisal firm Knight Frank. “The appeal of trophy real estate to foreign buyers is easy to understand in light of the cheap dollar.”

Behind the Numbers

In finding apartments with high costs per square foot, we examined the top tier of 10 markets across the country. The apartments we surveyed were usually not the most expensive on the market, but in the luxury stratosphere–as a high dollar numerator tended to have more of an effect on cost per square foot than a low square footage denominator. It doesn’t mean the homeowner paying a $1 million for a 400-square-foot Manhattan studio has any less of a gripe, just that he can’t lay claim to most expensive apartment per square foot.

Of course, with all real estate transactions, it’s best to do your homework.

“[Price per square foot] can be a good indicator of value, but it’s a slippery slope because you don’t always know what’s included,” he says. “From a new construction standpoint you can make some overall statements because it’s easier to find identical or very similar properties, but even so, buildings of the same size across the street from each other can be very different in price.”